An olefin (e.g., propylene) is commonly polymerized using an olefin polymerization catalyst. In particular, a propylene-based block copolymer produced using an olefin polymerization catalyst is widely used since rigidity and impact resistance are achieved in a well-balanced manner.
In particular, a propylene-ethylene block copolymer is used for a wide range of applications since the propylene-ethylene block copolymer exhibits excellent mechanical properties (e.g., rigidity and heat resistance), and can be produced relatively inexpensively.
A propylene-ethylene block copolymer is a blend of a polymer component that mainly includes propylene, and a random copolymer component obtained by copolymerizing propylene and ethylene, and is normally produced by sequentially effecting polymerization under the conditions that correspond to each component to blend the components within the reaction vessel.
For example, a propylene-based block copolymer that is obtained by performing a first step that effects homopolymerization of propylene, or random copolymerization of propylene and a small amount of ethylene, and performing a second step that effects copolymerization of propylene and ethylene, or propylene and another α-olefin, is widely used. The resulting propylene-based block copolymer may be melted, formed (molded) using a forming (molding) machine, a stretching machine, or the like, and used for a variety of applications (e.g., automotive part, home appliance part, container, and sheet).
A solid catalyst component that includes magnesium, titanium, an electron donor compound, and a halogen atom as essential components is known as a component of the olefin polymerization catalyst that is used to produce a propylene-based block copolymer. A number of olefin polymerization catalysts including the solid catalyst component, an organoaluminum compound, and an organosilicon compound have been proposed.
Technology that produces a propylene-based block copolymer is required to produce polypropylene that exhibits high stereoregularity that affects rigidity in order to obtain a propylene-based block copolymer that exhibits rigidity and impact resistance in a well-balance manner, achieve high copolymerization activity in the second step in order to improve impact strength, achieve high randomness with respect to the ethylene distribution and the like in the copolymer, and ensure excellent polymerization controllability through high polymerization sustainability.
A propylene-ethylene block copolymer is widely used to produce an automotive bumper and the like by means of injection molding. Therefore, technology for producing a propylene-ethylene block copolymer having an improved melt flow rate (MFR) has been desired in order to improve the productivity of the injection molding process.
The MFR of a propylene-ethylene block copolymer is uniquely determined by the MFR of the propylene polymer component, the MFR of the propylene-ethylene random copolymer component, and the content of the random copolymer component in the block copolymer. It is necessary to adjust the MFR of the random copolymer component and the content of the random copolymer component to be equal to or larger than specific values in order to improve the impact strength of the propylene-ethylene block copolymer. Since it is desired that the majority of ethylene included in the propylene-ethylene block copolymer be incorporated in the random copolymer, and the amount of crystalline polyethylene be small, technology has been desired that ensures that relatively high polymerization activity is achieved when forming a propylene-ethylene random copolymer (rubber part) (as compared with the polymerization activity when forming polypropylene), and ethylene is efficiently introduced into the random copolymer, and an olefin polymerization catalyst and the like that exhibit relatively high random copolymerization activity have been desired.
A propylene-ethylene block copolymer that is used to produce an automotive bumper and the like has been required to exhibit improved impact strength (particularly improved impact strength at a low temperature). The impact strength at a low temperature depends on the brittle temperature of the random copolymer component. Since the brittle temperature increases when the propylene content in the random copolymer component is too high, and the impact strength at a low temperature becomes in sufficient, it is desirable to decrease the brittle temperature of the random copolymer component by increasing the ethylene content in the random copolymer component.
At present, a propylene-ethylene block copolymer is mainly produced using a gas-phase process. In particular, a gas-phase process that removes the heat of polymerization by utilizing the latent heat of liquefied propylene is considered to be advantageous in that high heat removal performance can be achieved using small-scale equipment.
A method for producing a propylene-ethylene block copolymer using a gas-phase method, wherein a polymer component (a) that mainly includes propylene is produced in the first polymerization step, and a propylene-ethylene random copolymer component (b) is produced in the second polymerization step (see above), has been proposed.
According to this method, however, when the residence time distribution of the polymer particles that have been obtained by the first polymerization step and are subjected to the second polymerization step is wide, the reactor used for the second polymerization step may be fouled, or the impact strength of the block copolymer (product) may decrease.
It is considered that such a problem occurs since the activity of the polymer particles that are subjected to the second polymerization step varies to a large extent due to the wide residence time distribution, and the amount of particles that produce the random copolymer component in the second polymerization step increases to a large extent. Therefore, it is necessary to use a production method that ensures that high polymerization activity is achieved during random copolymerization, the residence time is short, and the residence time distribution is narrow.
Since polypropylene is normally produced using hydrogen that undergoes a chain transfer reaction as a molecular weight modifier, it is necessary to use hydrogen at a high concentration in order to produce polypropylene having a higher MFR (i.e., lower molecular weight).
Therefore, when producing polypropylene having a high MFR using a gas-phase process that utilizes the latent heat of liquefied propylene, there is a tendency that the hydrogen concentration in unreacted propylene gas increases, and the dew point of propylene decreases since hydrogen is used at a high concentration. As a result, productivity decreases due to removal of the heat of polymerization. When producing a random copolymer component having a high comonomer content using a comonomer having a low dew point (e.g., ethylene), the comonomer concentration in unreacted gas increases since the comonomer is used at a high concentration, and the heat removal performance in the recycle system becomes insufficient.
Specifically, when producing a propylene-ethylene block copolymer having a high MFR and a high ethylene content, insufficient heal removal or a decrease in productivity easily occur in the first polymerization step due to a high hydrogen concentration, and insufficient heal removal or a decrease in productivity easily occur in the second polymerization step due to a high ethylene concentration. In order to solve these problems, it is desirable that polypropylene having a high MFR can be produced at a lower hydrogen concentration, and a random copolymer component having a high ethylene content can be produced at a lower ethylene concentration.
Several polymerization catalysts that solve the above problems have been proposed.
For example, a method that improves the hydrogen response by utilizing an aluminum halide when producing a solid catalyst (see Patent Literature 1), a method that utilizes an organoaluminum component and an organozinc component in combination as a promoter (see Patent Literature 2), a method that utilizes an organosilicon compound that includes an amino group (see Patent Literature 3, for example), and the like have been proposed as a method for producing polypropylene having a high MFR.
A method that utilizes a titanium compound that includes a Ti—N linkage (see Patent Literature 4), a method that utilizes an organosilicon compound and a saturated hydrocarbon during second-step polymerization (see Patent Literature 5), and the like have been proposed as a method that solves the problem with regard to the copolymerizability of ethylene.
A method for producing a propylene-ethylene block copolymer, wherein an oxygen-containing compound or the like that is gaseous in a normal state is added when effecting second-step polymerization in order to suppress adhesion between polymer particles and adhesion of polymer particles to the inner wall of the reactor, has been proposed (see Patent Literature 6, for example).